to use conductance. Did you see my other post on using the washing machine
solution? You probably couldn't find water more contaminated than in a
clothes washer, and the diaphram method can work for decades without
failure.

Your proposed solution of the level-sensing problem with a washing
machine sensor is rather failproof.
But all the different drawbacks of a conduction based sensor made me
think of capacity change. This can be enormous, given the very high
value of the dielectric constant of water.
And as the original poster seems to want to go rather far,
technologically speaking, by using a microcontroller...